Septic tanks are used for primary treatment of wastewater, usually in connection with on-site subsurface systems that service residential dwellings and smaller commercial properties. Larger modern plastic septic tanks typically have two, top openings that are big enough for a person to enter into the tank. Typically such openings are at least 16 inch in diameter, more often 20 to 24 to 30 inch in diameter. Septic tanks are commonly buried in soil and a lid is secured to the opening(s) at the top of the tank, either directly or with an interposing riser.
A septic tank is desirably maintained every several years, particularly by suctioning through a hose the accumulated sludge proximate the tank bottom. To do that, the lids are removed and a flexible suction pipe line of 3-4 inch diameter is inserted into the tank opening, so it extends to the bottom of the tank. While that may suggest that only a small access opening might be sufficient, in most commonly available septic tanks the above-mentioned “man-size” openings are provided. One reason to have the large openings is that they enable man-access during manufacturing, for instance to install a baffle that divides the tank into fore and aft sections. Another reason is to enable relatively rare but important maintenance where access is needed to the interior, for such as to replace a baffle or fittings at the inlet or outlet ends of the tank. Tanks used for other purposes than wastewater treatment can present analogous needs which require an opening large enough to enable a human to enter the interior.
Most curious passing-by people, especially children, will not have sufficient tools to enable removal of the screws which typically secure a lid to the top of a tank or riser, if and when the lid is exposed. (In some locales it is increasingly preferred that the lids be above the surface of the soil.) In the course of maintenance and lid removal, the screws may be lost when removed; or through inattention or carelessness workers may fail to replace the screws. An unsecured lid, being light in weight, can be a tempting target for removal by curious people. It has been appreciated that an unfastened or otherwise easily removable lid is a potential hazard for a child, who could fall through the opening and into the water of the tank, to possible grievous end.
One option to addressing the problem might be to use screws or other kinds of fasteners of the lid which cannot be “lost,” that is, fasteners which are trapped in the openings of the rim of the lid by locking rings. However, any such fastening system has to be easily used in the sometimes-poor cleanliness and weather conditions which attend septic tank maintenance, and has to be itself reliable and durable. Another option is to make the lid heavy, as by using a concrete lid like those used on old concrete septic tanks, or by adding weight to a plastic lid, as taught by Lombardi II U.S. Pat. Pub. 2005/0178721. Another preventive measure has been to put a net or grid within the large opening of a septic tank port, the device having openings large enough to enable pump-out line passage, but small enough to inhibit pass-through of a child. See for instance Koteskey U.S. Pat. Pub 2004/261343 and Gavin U.S. Pat. Pub. 2016/0122126.
Approaches which increase complexity or cost run counter to the aim of keeping plastic septic tank costs low, in context that plastic tanks are often competing in the marketplace with concrete tanks. There is a need for another approach to making septic tank lids “child-resistant” has to be low cost, reliable, and durable.